[Kotlin] select date library

##### Description

select date library like Java Generator.

##### Swagger-codegen version

2.2.3+

##### Related issues/PRs

https://github.com/swagger-api/swagger-codegen/issues/5730

##### Suggest a fix/enhancement

`generate -i swagger.yaml -l kotlin  -o out -DdateLibrary=threetenbp`